1. Field of the Invention
The present invention relates to a parallel flat type plasma CVD apparatus.
2. Description of the Related Art
In the fabrication of thin-film semiconductor devices, a parallel flat type plasma CVD apparatus is used to form thin films. FIG. 1 presents a cross-sectional view of a conventional plasma CVD apparatus. Provided on the upper inner wall of a reaction chamber 5 is a heater panel 4 with its heating surface set horizontally. A metal holding electrode 2 is provided below the heater panel 4 in the reaction chamber 5. A glass substrate 1 is attached to this holding electrode 2.
The holding electrode 2 has an open center in which a window 2a for placing the glass substrate 1 is provided, as shown in FIG. 2. A small projection is provided at the lower portion of the window 2a to be used to mount the glass substrate 1 in the window 2a. A uniform heating plate 3 is placed on the glass substrate 1 and is secured to the holding electrode 2 by means of a fastener 9.
A gas guiding member 7 for guiding, for example, SiH.sub.4 and H.sub.2 into the reaction chamber 5 is mounted on the bottom wall of the reaction chamber 5 via an insulating member 5a. This gas guiding member 7, made of a conductive material, has a top end portion extending horizontally with multiple gas outlet holes 6a formed in the top of the top end portion. The portion where the gas outlet holes 6a are formed constitutes a high-frequency electrode 6. This high-frequency electrode 6 faces the holding electrode 2 under the holding electrode 2.
A high-frequency power supply 11 is connected to the gas guiding member 7 so that high-frequency power having an oscillation frequency of 13.56 MHz is applied via the gas guiding member 7 to the high-frequency electrode 6 from the high-frequency power supply 11.
A gas discharge port 8 is provided in the bottom wall of the reaction chamber 5, which is grounded.
According to the thus constituted conventional plasma CVD apparatus, when a raw gas is supplied into the reaction chamber via the gas guiding member 7 while the glass substrate 1 is heated by the heater panel 4, and high-frequency power is applied between the high-frequency electrode 6 and the holding electrode 2 and glass substrate 1 by the high-frequency power supply 11, an amorphous silicon film is grown on the glass substrate 1.
When a thin film is formed by this conventional plasma CVD apparatus, the peripheral portion of the glass substrate 1 is affected by the holding electrode 2 because of the difference between the surface potential of the glass substrate 1 and that of the holding electrode 2. As a result, the film thickness changes from the center portion of the substrate to its periphery, and the film thickness is not uniform.
As a solution to this problem, a technique has been proposed by which an insulating member is attached, by means of screws or the like, onto the surface of the electrode (holding electrode) located on the side where the insulator substrate is to be attached, thus ensuring a uniform electric field to provide improved uniformity of the film thickness (see Unexamined Japanese Patent Publication No. Sho 62-189725). Unexamined Japanese Patent Publication No. Sho 61-177374 proposes another technique, by which an insulating ring is attached to the peripheral portion of the electrode located opposite the substrate, for uniform plasma distribution in the discharge area and improved uniformity of the film thickness.
The former method that attaches an insulating member to the electrode surface in order to make the electric field of the entire substrate uniform at the time of forming the thin film, thus improving the film thickness distribution, also forms a film on this insulating member, which may produce particles. This method therefore requires regular cleaning of the insulating member. At the time the insulating member is cleaned, the film formed on the surface of the insulating member must be removed by a plastoprocess and etching or the like and the insulating member must then be secured again to the electrode surface by screws. It is apparent that this method requires complicated maintenance work.
Like the former method, the latter method of attaching the insulating ring to the peripheral portion of the electrode located on the side opposite the substrate also requires complicated maintenance work.